The endowment effect is a hypothesis that people value a good more once their property right to it has been established. In other words, people place a higher value on objects they own relative to objects they do not. In one experiment, people demanded a higher price for a coffee mug that had been given to them but put a lower price on one they did not yet own. The endowment effect was described as inconsistent with standard economic theory which asserts that a person's willingness to pay (WTP) for a good should be equal to their willingness to accept (WTA) compensation to be deprived of the good. This hypothesis underlies consumer theory and indifference curves.

The existence of the effect has been questioned by some economists. Hanemann (1991) noted that economic theory only suggests that WTP and WTA should be equal for goods which are close substitutes, so observed differences in these measures for goods such as environmental resources and personal health can be explained without reference to an endowment effect. Shogren et al (1994) noted that the experimental technique used by Kahneman and Thaler (1990) to demonstrate the endowment effect created a situation of artificial scarcity. They performed a more robust experiment with the same goods used by Kahneman and Thaler (chocolate bars and mugs) and found no evidence of the endowment effect.

Whether or not the endowment effect is a relevant economic phenomenon is somewhat uncertain; it is possibly a reflection of conventional substitution effects.